1. Field of The Invention
The present invention relates generally to electrodeless light sources and, more particularly, to a method and apparatus for shielding electromagnetic interference generated by an electrodeless fluorescent light source for a backlit display.
2. Background of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Conventional fluorescent lamps are driven with an electronic ballast which powers the lamps via electrodes disposed at each end of the lamp. The electrodes, however, are major life-limiting components of the fluorescent lamp. Electrodeless fluorescent lamps also are known. An electrodeless lamp is configured as a closed loop tube around which one or more coupling transformers are positioned. As with conventional fluorescent tubes, the electrodeless lamp is energized by an electronic ballast. However, rather than applying power to electrodes disposed at each end of a lamp tube, the ballast drives the coupling transformers, which, in turn, inductively couple the power to the lamp. The elimination of electrodes from the fluorescent lamp is particularly advantageous as it increases the life and reliability of the lamp and systems incorporating such lamps. Thus, electrodeless lamps are particularly useful in applications in which access to the lamps is restricted such that replacement of the lamps becomes difficult or expensive.
Backlit video display devices are one type of application in which the access to the lamp is not readily available. Such video displays may be found in computer systems, automatic teller machines, information kiosks, gas pumps, shipboard controls, etc. To enhance viewing of displayed images, such video displays commonly include a backlight source to provide a brightly lit background that contrasts with the displayed image. However, such video displays often are located in environments in which the ambient lighting conditions vary considerably, interfering with vivid viewing of the displayed image. For example, in a dimly lit environment (e.g., a cloudy day, the enclosed interior of a ship, etc.), a brightly lit background provides for the best viewing of a displayed image. However, in a brightly lit environment (e.g., a sunny day, a well-lit office, etc.), a dimly lit background provides for better viewing. Unfortunately, electrodeless lamps rarely are used in such displays due to the lack of suitable means for dimming such lamps. Accordingly, it would be desirable to provide the capability to control the brightness of the backlighting to compensate for variations in ambient lighting to enhance the viewing capabilities of the video display unit further.
The use of electrodeless fluorescent lamps is not limited to backlight sources for video display units, such as a liquid crystal display (LCD), or applications in which the lamp is not readily accessible. However, when used as a backlight source, the inventor has noted that video artifacts (e.g., dark horizontal stripes or bars) may appear on the display when the lamp is energized. The artifacts are believed to result from the effects of a stray magnetic field generated by the AC lamp current when the lamp is energized. These artifacts, which may detract from or otherwise impair viewing of an image, are particularly noticeable when dimming circuitry is incorporated into the display system.
Dimming of such display systems generally may operate via the implementation of circuitry which varies the energy provided to the electrodeless lamp. For example, such circuitry may energize the lamp at an adjustable duty cycle using a pulse width modulation scheme. As a result of the pulse width modulation, the AC current through the electrodeless lamp flows in bursts and generates corresponding bursts of the stray magnetic field. The inventor has observed that when the lamp is positioned near the display panel during the time interval when the AC lamp current is flowing, video artifacts on the display panel (e.g., dark horizontal stripes) are particularly visible. Accordingly, it would be desirable to provide an electrodeless lamp source for a backlit display system including a feature which shields the display panel from the magnetic field generated when AC current is flowing in the electrodeless lamp, particularly for a system which incorporates a dimming feature.
The present invention may address one or more of the problems set forth above.
Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.
In accordance with one aspect of the present invention, an electrodeless light source includes an electrodeless lamp which generates a lamp magnetic field when energized, and a shield device disposed proximate the electrodeless lamp and configured to oppose the lamp magnetic field.
For example, in an exemplary embodiment, the shield device comprises a closed conductive loop. The conductive loop is positioned with respect to the lamp such that the lamp magnetic field causes a shield current to flow in the closed loop. The shield current, in turn, causes generation of a shield magnetic field substantially opposes the lamp magnetic field.
In accordance with another aspect of the present invention, a backlit display device includes an electrodeless lamp to generate light, a power source to energize the lamp, a display unit to display an image, and a shield device. The lamp is operably positioned with respect to the display unit to illuminate the display unit to enhance viewing of the image. When the lamp is energized, AC lamp current flows therethrough and causes generation of a lamp magnetic field that may cause effects, such as visual artifacts, that may impair viewing of the image on the display unit. The shield device is operably positioned with respect to the lamp and the display unit to reduce the effects of the lamp magnetic field on viewing of the image.
In accordance with a further aspect of the present invention, there is provided a method for reducing an effect of a first magnetic field on a display unit having an electrodeless backlight source. The backlight source includes an electrodeless lamp that produces the first magnetic field which the lamp is energized. The method comprises inductively coupling energy to the lamp such that an AC lamp current flows in the lamp when the lamp is energized and generating light. The method further includes generating a second magnetic field that substantially opposes the first magnetic field. In an exemplary embodiment, generating the second magnetic field is accomplished by inducing a shield current in a shield device that is operably positioned with respect to the lamp such that the second magnetic field substantially opposes the first magnetic field.